破壊発生における延性 : 脆性遷移と亀裂端の変形挙動との関連について

概要

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The effects of specimen thickness on the fibrous/cleavage transition in the mode of fracture initiation were investigated. Tests were made on five low and medium strength steels, using 3 point bend specimens. The temperature at which the above transition occurs increases with the increase in specimen size and takes the upper limiting value when the plane strain state develops at the notch tip in the midthickness region, which was confirmed from observing the distribution of the stretched zone depth along pre-crack front. A region of constant stretched zone depth, whose deformation state was assumed to satisfy plane strain conditioh, was observed in the midthickness region in the specimen which showed the upper limiting; transition temperature, while in other cases the stretched zone depth usually showed convex parabolic distribution. As the specimen size requirement to give this upper limiting value of the transition temperature in the mode of fracture initiation, the formula a,B, W-a>0. 4 (δ_I)/(σ_YR) is proposed, where a is crack length, B thickness, W-d uncracked ligament, _i COD at fibrous crack initiation, _Y yield stress, and R the ratio of the yield strength to the ultimate tensile strength.
社団法人日本船舶海洋工学会の論文